1. Field of the Invention
The present invention relates generally to a process for forming polymers comprising alkyl or aryl groups linked through oxygen, nitrogen or sulfur atoms, and, more particularly, to a photochemical vapor deposition process for depositing thin layers of such polymers on a substrate.
2. Description of the Background Art
Dielectric or insulating materials such as silicon dioxide and silicon nitride are widely used in the fabrication of semiconductor devices and circuits to provide a layer of electrical insulation which prevents contact or unwanted current flow between adjacent conductive areas or materials. In addition, such insulating materials are used as a surface passivation layer to protect the surface of the substrate or as a mask during such procedures as etching or ion implantation. Such layers have typically been formed by thermal chemical vapor deposition at elevated temperatures, by plasma-enhanced chemical vapor deposition, or by reactive or nonreactive sputtering techniques. However, the latter three processes have the disadvantage of producing charge bombardment or radiation bombardment of the substrate and consequent degradation of device performance, while the thermal process produces thermal damage to the substrate. A recently developed process for oxide deposition which overcomes these disadvantages is the photochemical vapor deposition process described in U.S. Pat. No. 4,371,587, assigned to the present assignee. Similar photochemical processes for depositing nitride layers and sulfide layers are described in U.S. Pat. Nos. 4,181,751 and 4,447,469 respectively, both assigned to the present assignee.
As an alternative solution to the previously discussed difficulties in depositing oxide or nitride layers by conventional processes, as well as the problem of inadequate step coverage using such processes, it has been suggested that certain polymers be used as insulators in multilayer structures, as described, for example, by S. J. Rhodes in the publication entitled "Multilayer Metallization Techniques for VLSI High Speed Bipolar Circuits", in Semiconductor International, March 1981, pages 65-70. Rhodes describes a method of forming a layer of a polyimide on a metal layer by applying the polyimide in liquid form using conventional photoresist spin coating techniques and then curing the the material by a high temperature bake for imidization at 400.degree. C. However, the elevated temperatures required in such processes in order to polymerize the monomer produce unwanted side-effects, such as the formation of hillocks and spikes in the underlying metal layer which, in turn, produces pinhole defects in the overlying insulator layer. In addition, elevated temperatures can produce boundary migration of defined regions in a semiconductor device and can result in degraded device performance and decreased device yield. Furthermore, processes requiring elevated temperatures are unsuitable for use on temperature-sensitive materials, such as certain plastics or mercury cadmium telluride and other compound semiconductor materials which decompose at temperatures as low as 120.degree. C.
Thus, the need exists for a low-temperature process for depositing a layer of a polymer insulator on a substrate in the fabrication of semiconductor devices and circuits so that thermal damage to the substrate is avoided, while at the same time a good quality insulator layer is produced.